This invention relates to the electrodeposition of metallic coatings, and, more particularly, to the electrodeposition of amorphous alloys.
Metals normally exist in the crystalline state at ambient temperature, with the atoms of the metallic crystal arranged in a lattice having a periodically repeating structure. Metals can also exist in the amorphous state at ambient temperature. In the amorphous state, a metal has no crystallographic structure or lattice, and there is no short range or long range repeating order to the metallic structure. There is also no grain structure in amorphous metals, inasmuch as grains are a direct result of the presence of a crystalline structure.
Some amorphous materials can be made extremely hard and wear resistant, while at the same time highly corrosion resistant because of the absence of preferred orientations, grain boundaries and other defects. Additionally, some very hard and wear-resistant amorphous materials may also have considerably greater ductility than that of crystalline materials of comparable hardness and wear resistance. Even a few percent of ductility in such a hard, wear-resistant material can be highly significant, inasmuch as one drawback of many such materials is their tendency to crack during fabrication, use, or temperature cycling. When the material cracks, particularly if the wear-resistant material is used as a coating, its effectiveness may be lost, as the continued wearing action tends to remove flakes of the coating by spalling. Amorphous materials offer the potential of combining wear and corrosion resistance with sufficient ductility to prevent cracking and spalling, presenting attractive design possibilities in avoiding wear damage to other materials, as by the application of an amorphous wear-resistant coating.
Metals typically form from the liquid state as crystals, and special care must be taken to produce the amorphous state, when that state is desired. It has long been known that amorphous metals may be prepared by cooling a liquid metal of appropriate composition very rapidly from the liquid to the solid state. (See, for example, U.S. Pat. No. 3,297,436.) When a metal having the ability to exist as an amorphous structure, known as a glass former, as quenched from the liquid state at a cooling rate on the order of 10.sup.5 .degree. C. per second or greater, an amorphous structure is formed. Various types of apparatus have been developed to produce rapidly quenched amorphous materials as ribbons or powders. More recently, it has become possible to produce amorphous structures by passing a high intensity heat source over a crystalline structure of appropriate composition, so that the surface of the crystalline structure is melted and rapidly cooled against the remaining metal as a heat sink, thereby producing an amorphous surface structure. Lasers or electron beams may conveniently be used as the high intensity heat source.
All of the techniques for producing amorphous metals utilizing a high cooling rate from the liquid state have advantages in certain instances, but in other situations cannot be used to produce an amorphous structure. For example, it would be desirable to deposit a protective amorphous layer having high wear resistance and acceptable ductility on the inside surface of a cylindrical bore, as for example in producing a highly wear-resistant cylinder housing or pump housing bore. Fabrication techniques utilizing specialized apparatus employing a high cooling rate cannot be readily used to fabricate such a structure.
A promising alternative approach to producing amorphous metals is electrodeposition. Under the proper conditions of bath composition, voltage and current parameters, an amorphous layer may be deposited on a cathode by electrodeposition. For the most part, the electrodeposition of amorphous alloys has been limited to a few demonstration systems of little direct practical interest, and there are no known instances of the electrodeposition of high-hardness, wear-resistant, moderately ductile amorphous alloys. If a technique could be found to produce such materials it would then be possible, for example, to produce highly wear-resistant barrel liners by replacing the conventional low-ductility chromium cylinder liner coating with an amorphous layer that would resist spalling of the coating. Spalling often is observed following repeated thermal and stress cycles of a barrel having a chromium cylinder liner coating. Many other such applications may be envisioned, including, for example, pump housings, instrument bores, piston rings, cylinder housings, bearings, and bearing races.
Thus, there is a need for a process for preparing coatings of high-hardness, wear-resistant, moderately ductile amorphous alloys by electrodeposition. Inasmuch as it has been previously observed that many such amorphous materials contain metalloids such as boron, the process will desirably allow the electrodeposition of a boron-containing amorphous alloy directly from an aqueous electrodeposition bath under conditions that are sufficiently reproducible and forgiving of minor processing variations that the electrodeposition process may be used commercially and to produce relatively large coated structures. The present invention fulfills this need, and further provides related advantages.